Rescuing a Stuck Ferment
If you make your own wine there's a fair chance that sooner or later you are going to encounter that irritating problem - 'a stuck ferment'. This is when fermentation just stops part way through for no immediately apparent reason, and you’ve got a gallon (or more!) of low alcohol, over-sweet, not-quite-wine. It isn’t usually a major problem, and it rarely involves chucking the lot down the sink, but it can sometimes require a bit of work and thought to get things back on track. So what exactly is a stuck ferment? Before we answer that, it’s worth looking at:
What isn’t a stuck ferment?
There are two common reasons why fermentation stops in a way that cannot be called a stuck ferment, and they are both to do with sugar, plus a third that is not to do with sugar.
1) Too much sugar. If you had too much sugar (or other fermentables) in the original must, then the yeast may well do all the fermenting that it can do (which can be as much as 18 or 19% alcohol with some modern yeasts) but there is still sugar left in solution, and the yeast is finished. In this case, you’ve got a strong, sweet wine. You can either enjoy it as it is, or try blending it with an overly dry wine. Take a look at Paul's blog that covers blending, "Phil Says That's $^&*$@G Horrible!"
2) Not enough sugar. If you didn’t have enough sugar to start with, then the yeast will stop working when it runs out of food. The wine will be dry but low in alcohol. This is usually easily fixed by simply adding more sugar, and the yeast will wake up again.
These two problems can be easily avoided by using a hydrometer to gauge and keep track of the sugar content of your wine. This really is an essential piece of equipment for wine and beer making. Take a look a Paul's blog "Specific Gravity and Calculating % Alcohol by Volume".
3) Your wine has gone off. That is, it has got a bacterial infection that has turned it to vinegar, or caused other major problems that are often not easily, if at all, fixable.
So, what is a stuck ferment?
There is still sugar available for the yeast. The alcohol is not too high - in fact it’s probably nowhere near finished. (Your hydrometer readings have proved those two points to you). You’ve had a sniff and a taste and it’s obviously not gone off. And yet fermentation has stopped. There are no bubbles coming through the airlock, in fact there are no bubbles visible in the wine, and the yeast may be showing signs of settling out. What is going on? You’ve got a stuck ferment, that’s what.
Here is the Brew2Bottle guide to stuck ferment causes and remedies, listed in order from easiest to trickiest to fix. If you’re not sure what is causing your problem then we’d suggest starting at the top of the list and working your way down. By way of encouragement - it is very, very rare that a stuck ferment can’t be restarted.
1) Sometimes the easiest thing is to just give it a really vigorous stirring or shaking. Possibly just introducing a bit of oxygen, or releasing an excess of dissolved carbon dioxide, or dissolving excess sugar (see point 4, below), is enough to wake the yeast up.
2) It’s too cold. If the temperature of the wine is regularly dropping below about 15°C, that can cause the yeast to get a bit stroppy. Try to maintain a regular, non-fluctuating temperature, or consider some temperature control, such as a heat panel or immersion heater.
3) It’s too hot. Avoid putting your wine in airing cupboards or direct sunlight, or next to radiators. This is often worse than too little heat. A cold yeast may well just go dormant, and reactivate when the temperature increases. Overheating can cause the cell walls of the yeast to rupture, and the yeast itself to die, in which case you need to look at point 7, below.
4) Undissolved sugar can inhibit yeast. If you added a bit of extra sugar shortly before the fermentation stopped, that may well be the problem. As above, try giving it a good stir. If possible, when you want to add extra sugar, dissolve it in a bit of water to make a strong syrup rather than adding it neat.
5) Lack of nutrient. Yeast can survive with just sugar, but in order to flourish and produce plenty of alcohol it needs extra nutrients, much as we need vitamins and minerals. Things like potassium salts and nitrogenous compounds are very important, and are provided in yeast nutrients such as Tronozymol, and Vitamin B1 Tablets. A teaspoon or two of nutrient and a couple of B1 tablets added to the gallon will work wonders.
6) Lack of acid. Yeast also needs a slightly acidic environment in order to perform at its best. If you taste your stuck wine, and it seems a bit bland, a bit lacking in bite, then insufficient acid is probably the cause. The addition of a bit of acid, be it Citric acid, Tartaric acid, Malic acid, or a balanced mixture of all three will not only get your yeast up and running, but will also improve the taste and balance of your finished wine, and help with maturing and keeping. An addition of a small amount of Tannin will help as well.
7) You’ve tried all of the above and your wine is still just sitting there sulking. This is the worst case scenario, and there’s nothing else to do but bring out the big guns. That is, put some new yeast in. But that is not as easy as it sounds. Your stuck wine may well be about 7 or 8% alcohol, and if you just chuck in some fresh yeast, that level of alcohol will kill it stone dead. Yeast needs to gently ease itself into increasing levels of alcohol. So, the thing to do is to get some Gervin GV7 restart yeast. Mix it in a demijohn with about a cup full of water and an equal amount of your stuck wine, plus a bit of nutrient, fit an airlock, and put it somewhere warm to get started. Once you think it is fermenting well, add enough of the stuck wine to double the volume again. When that is going OK add some more stuck wine and repeat until the whole lot is fermenting properly.
8) Breathe a sigh of relief. You’ve just rescued a stuck ferment.
Now relax and look forward to your wine.
By Ade Bowen